This application claims the benefit of Korean patent application No. 97-1797, filed Jan. 22, 1997, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device having an in-plane structure (IPS) where a common electrode is formed parallel to a data line in the same plane, and more particularly, to a driving method where AC voltage is applied to the common electrode of an LCD with an IPS.
2. Discussion of the Related Art
A cathode ray tube (CRT) is the most widely used display device in television sets or computer monitors, because the CRT can easily reproduce the color and has a high response speed. However, the CRT is too large, too heavy, and requires too much power for portable applications. In order to overcome these disadvantages of the CRT, a great deal of research and development has been conducted into other types of displays. Among them, a liquid crystal display (LCD) is one of the most commonly used devices.
The LCD can be used as a thin television set mounted on a wall, because the LCD does not have an electron gun, unlike the CRT. Furthermore, the LCD can be used as a portable display device in a notebook computer, because the LCD""s power consumption is very low, and it can be driven by a battery.
In general, the LCD includes a liquid crystal panel 12 displaying the video data, and driver IC""s 10 and 11 for controlling video data, as shown in FIG. 1. The liquid crystal panel 12 includes, as shown in FIG. 2, a first substrate 25, a second substrate 21, and a liquid crystal layer 24 injected between the first substrate 25 and second substrate 21. The first substrate 25 includes a plurality of scan lines 14 and a plurality of data lines 15, with the scan lines 14 and the data lines 15 arrayed in a matrix. The first substrate 25 also includes a pixel electrode 26 (see FIG. 2) and a thin film transistor (TFT) 13, formed at crossing locations of the scan lines and data lines. Both the second substrate 21 and the first substrate 25 include a common electrode 23 and a color filter layer 22. The pixel electrode 26 and the common electrode 23, which face each other, and which have the liquid crystal between them, act as a pixel 16 shown in FIG. 1. The liquid crystal panel also includes a polarization plate 20 on outer sides of the first substrate 25 and the second substrate 21.
The TFT includes a gate electrode 30 (which is usually made of chromium), a source electrode 32, and a drain electrode 33 made of a transparent conductive material such as indium tin oxide, a semiconductor layer 34, and a doped semiconductor layer 36. The gate electrode 30 is connected to the scan line 14, and the source electrode 32 is connected to the data line 15. The drain electrode 33 is connected to the pixel electrode 26. The TFT works as a switch, which passes a data voltage applied to the data line 15 to the drain electrode 33 when a scan voltage is applied to the gate electrode 30 through the scan line 14. If the data voltage is applied to the drain electrode 33, then it is applied to the pixel electrode 26 connected to the drain electrode 33. Thus, an electric field exists due to a voltage difference between the pixel electrode 26 and the common electrode 23. The orientation of the liquid crystal molecules between the pixel electrode 26 and the common electrodes 23 rotate in response to the electric field. Thus, the amount of light transmitted at the pixel changes. That is, there is a difference in light transmittance between the pixel having a data voltage applied to it and the pixel without the data voltage applied. By using the pixels having the difference in transmittance, the LCD functions as a display device.
In this structure of the LCD, as shown in FIG. 2, the liquid crystal molecules rotate their orientation in a plane parallel to the orientation of the substrates. Therefore, a transmittance is highest in the tangential direction of the panel. However, the transmittance decreases as the viewing angle from the tangential direction increases. Thus, increasing the viewing angle is a very difficult problem for this LCD structure.
An in-plane structure (IPS) is one solution for increasing the viewing angle. In the IPS, as illustrated in FIG. 4a showing a plan view, a common electrode 23 of the pixel is parallel to a pixel electrode 26 and has a segment shape parallel to a data line 15. The bus lines for connecting the common electrodes 23 and the common lines 27 are parallel to the scan line 14. Referring to FIG. 4b showing the cross-sectional view of an LCD with IPS, the LCD includes a TFT having a gate electrode 30, a source electrode 32 and a drain electrode 33, the pixel electrode 26, and the common electrode 23 on the same substrate 25. The working principle of an LCD with IPS is the same that of a non-IPS LCD. However, the direction of an electric field is different from a non-IPS LCD. As shown in FIG. 5, the arrangement of the liquid crystal molecules 24 is parallel to the substrate, because the electric field is formed parallel to the substrate surface. Therefore, the liquid crystal molecules cut off the light or pass the light independent of the viewing angle.
Generally, driving methods for an LCD include line inversion, column inversion, or dot inversion. In the line inversion method, as shown in FIGS. 6a-6b, a polarity of a voltage applied to the pixel electrodes is reversed in every scan line. In the column inversion method, as shown in FIGS. 7a-7b, the polarity of the voltage applied to the pixel electrodes is reversed in every data line. In the dot inversion method, as shown in FIGS. 8a-8b, the polarity of the voltage is reversed for every row and column, that is, for every scan and data line.
In the line or column inversion method, a flicker problem is common. The reason is that when a scan line signal is high, all TFT""s connected to the scan line are turned on, and the data signals are sent to the pixel electrodes from the source electrodes, which are connected to the data lines. Then, the liquid crystal molecules are driven by the voltage difference between the pixel electrode and the common electrode. When the scan line signal is low, all TFT""s connected to the scan line are turned off. When that happens, the voltage applied to the pixel electrodes 26 remains on the pixel electrodes 26, the liquid crystal molecules remain in the same state of rotation, and display signals are maintained. However, the stored signal voltage in the pixel electrode is reduced somewhat (xcex94V) by the coupling capacitance (Cgs) formed between the scan lines and data lines. Thus, the liquid crystal display flickers because the voltages on the pixel electrodes 26 are not all the same.
In the dot inversion method the flicker problem does not occur because neighboring pixels have different signal values. As shown in FIGS. 8a-8b, if a positive signal is applied to a first pixel, a second pixel, which is a neighboring pixel, has a negative signal applied to it. At the next cycle, the first pixel has a negative signal and the second pixel has a positive signal applied to it. That is, the pixel signal has a pulse signal type, as shown in FIG. 9. The voltage differences (xcex94V), which occur in positive and negative states, can be moderated by control of the common voltage. Therefore, the voltage differences are the same, and the flicker problem can be solved.
In the dot inversion method, the voltage applied to the common electrode is a DC voltage, in general. On the other hand, to solve the flicker problem, the voltage difference should be maintained the same when the signal applied to the pixel electrode is AC. Thus, the power consumption of the dot inversion method is large, because the common voltage is a DC voltage. For example, if the voltage difference is 2.5V, and the common voltage is +2.5V, then the signal voltage applied to the pixel electrodes 26 is an AC voltage in the range from 5V to 0V. Therefore, in order to reduce the power consumption, the swing range of the voltage applied to the pixel electrode 26 should be reduced. If the common voltage is an AC voltage in the range between +1.25V and xe2x88x921.25V, and is 180 degrees out of phase with the signal voltage, then the signal voltage between +1.25V and xe2x88x921.25V is sufficient to maintain the 2.5V voltage difference. The method for applying an AC voltage to the common electrode 23 is known in the art. In this method, the common electrode is a patterned line parallel to the data lines.
Accordingly, the present invention is directed to a liquid crystal display in-plane structure and method of manufacturing the same that substantially obviates one or more of the problems due to the limitations and disadvantages of the related art.
An object of the present invention is to provide an IPS structure of an LCD that can be driven in an AC mode.
Additional features and advantages of the present invention will be set forth in the description which follows, and will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure and process particularly pointed out in the written description as well as in the appended claims.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in one aspect of the present invention there is provided a liquid crystal display device, including a substrate, a plurality of scan lines on the substrate, a plurality of data lines crossing the plurality of scan lines on the substrate, a plurality of pixel electrodes, each of the plurality of pixel electrodes formed near the crossing of a respective data line and a scan lines, a plurality of thin film transistors, wherein each of the plurality of thin film transistors includes a gate electrode connected to a corresponding scan line, a source electrode connected to a corresponding data line, and a drain electrode connected to a corresponding pixel electrode, a plurality of common electrodes each located near the corresponding pixel electrode, and a plurality of common lines connecting the corresponding common electrodes, wherein the plurality of common lines are parallel to the plurality of data lines.
In another aspect of the present invention there is provided a method of driving a liquid crystal display device with an in-plane structure mode, including the steps of sequentially applying a scan signal to a plurality of scan lines, applying a first data signal to a plurality of odd data lines, applying a second data signal having reversed polarity relative to the first data signal to a plurality of even data lines, applying a first common signal to a plurality of odd common lines, wherein the plurality of odd common lines are parallel to the odd data lines, and applying a second common signal to a plurality of even common lines, wherein the plurality of even common lines are parallel to the plurality of even data lines.
In another aspect of the present invention there is provided a method of forming a liquid crystal display device, including the steps of forming a scan line, a gate electrode, and a common electrode in a first layer on a substrate, forming a gate insulation layer over the scan line, the gate electrode, the pixel electrode and the substrate, forming a source electrode, a drain electrode, a data line, a common line, and a pixel electrode in a second layer, wherein the common line connects to the common electrode, and the data line connects to the source electrode, and forming a semiconductor layer over the gate electrode and the gate insulation layer.
In another aspect of the present invention there is provided a method of forming a liquid crystal display device, including the steps of forming a scan line, a gate electrode, and a pixel electrode in a first layer on a substrate, forming a gate insulation layer over the scan line, the gate electrode, the pixel electrode and the substrate, forming a source electrode, a drain electrode, a data line, a common line, and a common electrode in a second layer, wherein the drain electrode connects to the pixel electrode, and forming a semiconductor layer over the gate electrode and the gate insulation layer.
In another aspect of the present invention there is provided a method of forming a liquid crystal display device, including the steps of forming a scan line and a gate electrode in a first layer on a substrate, forming a gate insulation layer over the scan line, the gate electrode, and the substrate, forming a source electrode, a drain electrode, a common electrode, a common line, and a data line in a second layer and over the gate insulation layer, forming a semiconductor layer over the gate and the gate insulation layer, forming a protective layer over the semiconductor layer, the common electrode, the common line, the source electrode, the drain electrode, and the data line, and forming a pixel electrode in a third layer and over the protective layer, wherein the pixel electrode is selectively in contact with the drain electrode.
In another aspect of the present invention there is provided a method of forming a liquid crystal display device, including the steps of forming a scan line and a gate electrode in a first layer on a substrate, forming a gate insulation layer over the scan line, the gate electrode, and the substrate, forming a source electrode, a drain electrode, a data line, and a pixel electrode in a second layer over the gate insulation layer, forming a semiconductor layer over the gate electrode and the gate insulation layer, forming a protective layer over the source electrode, the drain electrode, the data line, and the pixel electrode, and forming a common electrode and a common line in a third layer over the protective layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.